1. Field of the Invention
This invention relates to an area light source apparatus called backlight which illuminates a liquid crystal display apparatus of the transmission type, which has a display area formed from pixels disposed in a two-dimensional matrix, from the back and a liquid crystal display apparatus assembly which incorporates the area light source apparatus.
2. Description of the Related Art
The liquid crystal material itself in a liquid crystal display apparatus does not emit light. Accordingly, for example, a direct area light source apparatus (backlight) for illuminating the display area of the liquid crystal display apparatus is disposed rearwardly of the display area which is formed from a plurality of pixels. It is to be noted that, in a color liquid crystal display apparatus, one pixel is composed of three sub pixels including a red light emitting sub pixel, a green light emitting sub pixel and a blue light emitting sub pixel. A liquid crystal cell which composes each pixel or each sub pixel is controlled so as to operate as a kind of an optical shutter (light valve) to control the light transmittance (numerical aperture) of the pixel or the sub pixel thereby to control the light transmittance for the illumination light (for example, white light) emitted from the area light source apparatus to display an image.
In the past, an area light source apparatus in a liquid crystal display apparatus assembly illuminates the entire display area with a uniform and fixed brightness. However, an area light source apparatus having a configuration different from that of such an area light source apparatus as described above, that is, an area light source apparatus including a plurality of area light source units and having a configuration for varying the distribution of the luminous intensity over the display area units, is in the past known as an area light source apparatus of the partial driving type or the divisional driving type and disclosed, for example, in Japanese Patent Laid-Open No. 2005-258403. Control of such an area light source apparatus as just described is called partial driving or divisional driving of an area light source apparatus. By such partial driving or divisional driving, increase of the contrast ratio by increase of the white level in the liquid crystal display apparatus and drop of the black level can be achieved. As a result, enhancement of the quality of image display can be achieved and reduction of the power consumption of the area light source apparatus can be anticipated.
A light emitting diode (LED) assembly is frequently used as a light source which composes an area light source apparatus. The light emitting diode assembly includes a plurality of light emitting diodes, and a plurality of lenses through which light emitted from the light emitting diodes passes. The light passing through each lens has a radiation angle characteristic (for example, a Lambertian distribution) based on the characteristic of the lenses. Then, the red light emitting diodes, green light emitting diodes and blue light emitting diodes are energized to emit red light, green light and blue light, respectively. The red light, green light and blue light are mixed to produce white light, with which the display area of the liquid crystal display apparatus is illuminated. It is to be noted that the area light source apparatus includes a plurality of light emitting diode assembly groups each including red light emitting diode assembles, green light emitting diode assemblies and blue light emitting diode assemblies. The number of red light emitting diode assemblies, the number of green light emitting diode assemblies and the number of blue light emitting diode assemblies in one light emitting diode assembly group and the array pattern of the light emitting diode assemblies are normally determined based on specifications of the area light source apparatus. In order to prevent occurrence of color unevenness or luminance unevenness, the light emitting diode assemblies are frequently gathered in a narrow region.